U.S. Pat. No. 3,557,867 (herein fully incorporated by reference) describes an innovative casting system known in the casting art as the "lost foam casting process". In the lost foam casting process a polymer foam, e.g., STYRAFOAM brand foamed polystyrene, "casting form" is positioned in a container called a "pour box" and surrounded by a gas permeable bed of heat resistant granular material. In practice, sand is believed to be almost always used and typically is specially selected and treated for this process. The "pour box" is a substantially upright, substantially open top container defining a contained volume. The container may have a porous sidewall or sidewall portion and/or a porous bottom wall or bottom wall portion. A generally tubular "casting means" is positioned to extend into the container so that one end of the casting means opens to and contacts the casting form. The casting means is positioned generally vertically and functions not unlike a funnel in directing molten metal to the casting form. Sand is then added to surround the casting means. The sand is then generally vibrated to pack it tightly around the casting form and to hopefully interlock the sand granules and keep them in position during the casting step itself. When molten metal is poured into the casting means it quickly flows down and displaces the polymer foam casting form. The molten metal heats the casting form, causing it to decompose and in fact substantially converting it entirely to gases that are vented through the sand and then out the top and through the porous sections of the side wall and bottom wall of the pour box. The molten metal promptly fills the space occupied by the casting form as the form decomposes and disappears. When the molten metal cools and hardens the casting should have precisely the shape of the casting form having attached thereto a "gate" formed by a little excess metal remaining in the casting means. The gate is generally subsequently removed from the solidified casting.
The lost foam casting process represents a significant advance in the art of casting metals and allows casting to close tolerances not unlike die casting and also allows casting of intricate shapes that are difficult or impossible to die cast. However this process is still in need of improvement in at least one regard. In the event that the gases produced can not escape through the sand rapidly enough, then the back pressure often will cause defects such as unsound portions in the casting and may even expel molten metal back up the casting means leaving undesired void spaces in the resulting casting or an incompletely filled form.
One means of addressing this problem that has been attempted is to utilize a pour box having partially or entirely porous walls and bottom and to apply a partial vacuum to the exterior thereof during casting. However, applying a partial vacuum like this can cause "burn-in" on the casting for those areas of the casting form that are nearest the walls and bottom of the pour box. Burn-in is the condition wherein molten metal moves beyond the space filled by the casting form and penetrates appreciably into the surrounding sand. Even when a vacuum strong enough to cause burn-in is applied, portions of the casting form farthest from the walls and bottom of the pour box can still generate deleterious back pressure as the polymer foam decomposes thermally.